Project Summary: Genomic instability underlies countless human diseases, thus understanding the mechanisms promoting genetic fidelity is critical. RNA Interference (RNAi) and DNA methylation are two chromatin-modifying activities that regulate eukaryotic genomes. Using a Schizosaccharomyces pombe (fission yeast) model, we have discovered that RNAi is essential to prevent deletions within repetitive genetic elements. Employing this system, we will define the mechanism by which RNAi ensures genomic integrity within the ribosomal DNA (rDNA) and telomeric loci, both composed of repeats. We will also test whether deletions occur by homologous recombination. To study DNA methylation, we will interrogate the S. pombe rDNA loci, recognizing that other organisms use DNA methylation to control both genetic imprinting and gene dosage at their rDNA. Finally, we will investigate the potential link between RNAi and DNA methylation at the rDNA. Experiments will use an in vivo recombination assay that we have pioneered, and benefit from the genetic, biochemical, and genomic advantages offered by the fission yeast system. These studies will be conducted by a pediatric physician-scientist with superb training in molecular biology and human immunogenetics. The candidate's education demonstrates a strong commitment to academic medicine, and his clinical appointment guarantees 80% protected research time. The Pi's career goal is to become an independent investigator in human chromatin biology, with a long-term plan to integrate this work with pediatric immunology and oncology. Mentorship will be provided by an exceptional scientist, whose career is built upon chromatin biology in yeast systems. The laboratory is state-of-the-art, has a strong publication record, and is well-funded. This research environment is buttressed by an educational program of journal clubs, lecture series, and regular meetings with both mentor and an advisory committee. Relevance: The human genome is extraordinarily complex, due in part to its repetitive nature, making maintenance of intact, unmutated genetic material a difficult biologic challenge. Changes to human DNA cause myriad maladies, including congenital birth defects and cancer;other syndromes of intact?yet dysregulated?genomes are also recognized. This project will explore the mechanisms used by advanced genetic organisms to preserve and regulate their genomes, so as to prevent these types of harmful events.